Field of the Invention
The invention relates to a process for sealing leaks in gas spaces and/or gas channels located between individual components of high-temperature fuel cells. The invention also relates to a fuel cell being sealed in accordance with the process.
High-temperature fuel cells, which are also known as solid oxide fuel cells (SOFCs), are suitable for converting not only hydrogen gas and carbon monoxide but also hydrocarbons, such as natural gas or propane storable in liquid form, with oxygen or oxygen from the air, because of their relatively high operating temperatures, which are in the range of from 800.degree. to 1100.degree. C. If water vapor is added to the fuel, then any development of soot at the high temperatures can be averted.
In known high-temperature fuel cells, solid electrolytes are used for temperature reasons. In that respect it is known to insert thin small solid electrolyte plates, essentially being formed of zirconium oxide and small added amounts of yttrium, between the electrodes. The electrodes, that is the anode and the cathode, are located on opposite sides of the electrolyte, and sintered to or onto it. For example, the anode may be formed of a porous cermet of nickel and zirconium oxide, and the cathode may be formed of an oxide compound of lanthanum, strontium and manganese. The electrodes cover both sides of the small electrolyte plate, except for a narrow peripheral region. So-called bipolar plates or end plates adjoin the two electrodes on the outside and they are formed of a material with good electrical conductivity and have supply channels, which are so-called grooved zones, for conducting the oxygen-containing gas to the cathode, for conducting the fuel to the anode and for removing the product of oxidation. Those usually plate-like components contact the electrodes and, with the edges of the grooves, they support the electrodes of the small electrolyte plates. Often they are provided with apertures on their edges for supplying and removing gas. In the region of those peripheral zones of the bipolar plates, the small electrolyte plates are surrounded by a frame of an electrically non-conductive material, which has the same thickness as the small electrolyte plates. That frame has apertures which are congruent with the apertures in the peripheral region of the bipolar plate. So-called window foils are inserted between the bipolar plates and the small electrolyte plate. Those foils have window-like apertures on the edges of which the free edges of the small solid electrolyte plates, that is those not occupied by electrodes, rest. The window foils also have apertures on their edges that are disposed congruently with the apertures of the bipolar plate. The window foils are of the same material as the bipolar plates. They have approximately the same thickness as the electrodes sintered to the small electrolyte plates and serve to join the small electrolyte plates, together with the electrodes and the frames surrounding them, to one another in a gas-tight fashion through the respective peripheral regions. At the same time the window foils seal off the gas spaces on the anode and cathode sides from one another and from the apertures in the frame, through the edge of the small electrolyte plates and through the frames that surround the small electrolyte plates. In known high-temperature fuel cells, the bipolar plates, the window foils, and the frame that surrounds the small electrolyte plates are soldered in gas-tight fashion to one another, with the interposition of a solder that melts at above the operating temperature.
The various gas spaces or gas channels on both sides of the electrolyte must be sealed off both from the outside and from one another in gas-tight fashion. That is necessary, among other reasons, in order to ensure that fuel and oxygen will not mix and form a chemical short circuit which reduces the overall efficiency and that moreover, because of hydrogen combustion, would lead to local overheating. Moreover, the sealing is also necessary in order to avert damage to the electrodes, which would otherwise be destroyed in the presence of the other respective reaction gas.
The article entitled "High-Temperature Solid Oxide Fuel Cell--Technical Status" in the Journal of Power Sources, Vol. 10, No. 1, 1983, pp. 89-102, very generally describes a process in which porous substrates are sealed in gas-tight fashion by means of the known processes of CVD and EVD. It has been demonstrated that during manufacture, even if that process is employed, leakage problems have repeatedly occurred in known high-temperature fuel cells. Particularly when a very great number of individual fuel cells are put together to make a fuel cell stack, with a great number of individual, serially as well as parallel-connected fuel cells, it is an extremely difficult problem if just a few fuel cells in the stack have leaks.